Dissolution Of Tungsten Research Articles

Enhancing the anodic dissolution uniformity of dendritic-structured refractory high entropy alloys using a split electrolytic cell with a deep eutectic solvent

The attainment of uniform anodic dissolution in dendritic-structured refractory high entropy alloys (RHEAs) during the electropolishing process is essential for their utilization in high-precision components. Nevertheless, the intricate microstructures of these materials impede uniform dissolution, resulting in suboptimal surface quality. In this work, based on a choline chloride-based deep eutectic solvent and suitable polarization parameters selected by linear scanning voltammetry, a split electrolytic cell (SEC) was proposed to alter the traditional integrated electrolytic cell (IEC) to enhance the anodic dissolution uniformity of dendritic-structured RHEAs. A much smoother surface with Ra of 0.233 μm and a smaller height discrepancy of 0.8 μm was obtained, compared to those processed in IEC at the same condition. The non-uniform anodic dissolution was mainly attributed to the cathode-forming (hydroxide ion (OH−), dissolution of tungsten (W) in the dendrite region under the attack by OH−, and the formation of by-product, i.e., the water molecule. The positive impact of SEC on dissolution uniformity could be ascribed to its role in inhibiting OH− from interacting with the anode, as well as to the enhancement of solution acidity on the anode side, which facilitated the dissolution of metal oxides. The suggested reaction pathway was subsequently validated through experiments with controlled water concentration and assessing of electrochemical kinetics. Finally, the effects of SEC under different parameters were demonstrated. This work provides valuable insight into the precise, micro/nanoscale manufacturing and green electrochemical surface finishing of RHEAs in the future.

Mechanism of Anodic Dissolution of Tungsten in Sulfate–Fluoride Solutions

Thin passive films on tungsten play an important role during the surface levelling of the metal for various applications and during the initial stages of electrochemical synthesis of thick, nanoporous layers that perform well as photo-absorbers and photo-catalysts for light-assisted water splitting. In the present work, the passivation of tungsten featuring metal dissolution and thin oxide film formation is studied by a combination of in situ electrochemical (voltammetry and impedance spectroscopy) and spectro-electrochemical methods coupled with ex situ surface oxide characterization by XPS. Voltametric and impedance data are successfully reproduced by a kinetic model featuring oxide growth and dissolution coupled with the recombination of point defects, as well as a multistep tungsten dissolution reaction at the oxide/electrolyte interface. The model is in good agreement with the spectro-electrochemical data on soluble oxidation products and the surface chemical composition of the passive oxide.

Oxidative Dissolution of Cemented Tungsten Carbides in Molten Sodium Carbonate by Addition of Copper(I) Oxide as Oxidizing Agent for Tungsten Recycling

Due to the monopolized supply of tungsten resource, it is important to efficiently recycle tungsten scrap for use as a secondary resource. The recycling of tungsten from cemented carbide tools by the molten carbonate method was investigated using simulated hard and soft scrap (carbide tool tips and WC powder, respectively). The oxidative dissolution of tungsten was examined in molten Na2CO3 under Ar–O2–CO2 atmospheres at 1173 K. Based on the immersion potentials of Cu, W, Co, C, and WC–Co, Cu2O was suggested to work as an oxidizing agent for tungsten dissolution. The oxidative dissolution rate for carbide tool tips with 12.8 mol% Cu2O addition reached 57 mg h−1 for the reaction time of 2.5 h, equivalent to 0.32 mm h−1. The decrease in the dissolution rate after 2.5 h was attributed to the decrease in the Cu(I) ion concentration in the melt and the inhibition of ion diffusion by the deposited metallic Cu. No violent reaction leading to explosion was observed, even for the oxidative dissolution of fine WC powder with a large surface area. Thus, this method provides significant safety improvements compared to the molten nitrate method.Graphical

Preparation of powder by electrical discharge erosion and sintering of ultrafine WC-5TiC-10Co alloy with high hardness

In this work, studies were carried out on obtaining a powder by electrical discharge erosion (EDE) of a WC-5TiC-10Co cemented carbide, sintering an ultrafine-grained alloy from the obtained powder, and studying the microstructures and properties of the obtained particles and alloys. We have established that with an increase in the pulse energy from 0.5 J to 10 J, the EDE productivity increases linearly. At the same time, energy consumption decreases and tends towards a constant value (3.4–4.2 kWt*h/kg). It has been established that an undesirable change in the carbon content in the resulting powder from the liquid pyrolysis is proportional to the product of the specific energy consumption and the difference in the mole fractions of carbon and oxygen in the liquid. Excess carbon was removed by heat treatment in the appropriate volume of CO2. It has been established that EDE results in a complete dissolution of tungsten in cubic carbide (Ti,W)C, which is retained in the formed particles. During further heat treatment and sintering, new plate-like ultrafine WC grains grow from a supersaturated (Ti,W)C solution. An increased concentration of tungsten is retained during sintering in ultrafine-grained grains of a supersaturated (Ti,W)C solid solution. By reducing the average diameter of the WC grains from 1.45 μm to 0.42 μm and increasing the carbide content (Ti,W)C, the hardness of the obtained alloys increases from 1410 HV in the original alloy to 1650 HV in the sintered alloy. Fracture toughness increases due to formation of lamellar WC grains from 12.9 mPa*m1/2 to 14.1 MPa*m1/2.

Investigation of powder bed laser fusion additive manufacturing of 93W, 5.6Ni, 1.4Fe tungsten heavy alloys

Tungsten exhibits a variety of interesting properties with significant potential for advanced applications but is currently limited by the difficulty of forming complex shapes for end-use parts. Additive manufacturing using powder bed laser fusion techniques may offer an alternative method of forming tungsten parts with greater design flexibility. To investigate this potential, two 93W, 5.6Ni, 1.4Fe tungsten heavy alloy (WHA) powders were densified using a Renishaw AM 400 laser melting system to >99.8% theoretical density and the resulting microstructures assessed. High densification was possible utilizing long exposure times to maximize melting at low laser powers, low to moderate laser powers to avoid overheating the FeNi phase, and small point distances, hatch distances, and layer heights to maximize remelting. SEM investigation of the resulting microstructure revealed two distinct phases: nearly pure W and ∼ 90% W-rich W-Ni-Fe. This was believed to be the result of complete melting of the NiFe powder and either dissolution of tungsten within the liquid NiFe matrix or partial melting of the tungsten powder. High cooling rates in the additive manufacturing process resulted in a tungsten-rich W-Ni-Fe matrix phase compared to traditional liquid phase sintered WHA alloys. Microhardness testing was conducted to begin assessment of resulting mechanical properties, with extremely high hardness values being measured when compared with expected literature values. Fine microstructure and high W content in the matrix phase contribute to these high hardness values, but further investigation is needed.

Water Corrosion of Tungsten Target for Accelerator-Driven Neutron Source.

The water corrosion of tungsten as a target material can affect the safe operation of accelerator-driven neutron source. This paper reported the corrosion behaviors of tungsten in ultrapure water and tap water for 7, 14, 21, 30 and 60 days. Moreover, ICP-MS, XRD, XPS, SEM-EDS and LSCM were used to analyze the components in solutions, crystalline structures, chemical compositions and surface morphologies. It was found that the dissolution of tungsten, due to corrosion, reached its maximum between 30 days and 60 days in both solutions. The cube-shape substance, CaWO4, was the main corrosion product after tungsten in tap water. The tungsten oxide was changed from WO3 to WO2 during the corrosion of tungsten in ultrapure water. Compared with tungsten in ultrapure water, tungsten in tap water had its surface completely destroyed, with a dense diamond shape. Therefore, based on the analysis from this study, the corrosion mechanisms of tungsten in ultrapure and tap water were revealed.

Efficient dissolution of tungstic acid by isopolytungstate solution based on the polymerization theory of tungsten

The rare metal tungsten is a very important strategic metal. The traditional tungsten ore alkaline process has many problems, including the generation of alkali residue, ammonia‐nitrogen wastewater, and industrial wastewater during ion exchange. The feasibility of decomposing scheelite with sulfuric acid has been confirmed due to the economic and environmental pressures faced by scheelite decomposition; however, extracting tungsten from the decomposition residue is a difficult process. This study was based on the polycondensation theory of tungsten, in which tungstic acid (H2WO4) rapidly dissolves in isopolytungstate solution during the acid adjustment process of sodium tungstate solution. Taking advantage of the different polymerization morphologies, a new idea in which tungstic acid selectively enters the solution after the polycondensation reaction between tungstic acid and isopolytungstate is proposed. In this work, the dissolution of tungstic acid in isopolytungstate solution under different experimental conditions were studied. The results indicated that isopolytungstate solution can undergo polycondensation reaction with tungstic acid, and the dissolution of W reached 0.37 g/g under suitable dissolution conditions. Additionally, the residue formed in sulfuric acid decomposition had a satisfactory leaching effect of tungsten in the isopolytungstate solution with a WO3 leaching efficiency of about 98.8%. Tungsten in the hydrochloric acid decomposition residue could also be extracted efficiently in the form of an isopolytungstate solution with a WO3 leaching yield >99%. This work proposes a cleaner and more efficient idea for the selective dissolution of tungsten from the acid decomposition residue of scheelite.

Solvometallurgical Process for the Recovery of Tungsten from Scheelite

The industrial state-of-the-art processes to extract tungsten from scheelite (CaWO4) require high pressures and temperatures. These flowsheets consume a large excess of chemicals (which are very hard to recycle) and generate up to 25 tons of high-salinity wastewater per ton of ammonium paratungstate product. In this work, a more sustainable conceptual flowsheet for the recovery of tungsten from high-grade (55.0 wt % W) and medium-grade (3.3 wt % W) scheelite ores was developed at the lab scale. Leaching of CaWO4 was tested in solutions of 37.0 wt % aqueous HCl in organic solvents (ethylene glycol, poly(ethylene glycol) 200, acetonitrile) and ionic liquids (Aliquat 336). The tungstate (HxWyOzn–) generated by the reaction between CaWO4 and HCl was only solubilized in the ethylene glycol system with an appropriate amount of 37.0 wt % aqueous HCl, whereas in all other solvents, it either precipitated or CaWO4 did not dissolve. After the dissolution of tungsten, nonaqueous solvent extraction was used to separate tungsten from calcium, by means of a solvent consisting of 20 vol % Aliquat 336 in the aliphatic diluent GS190 with 10 vol % 1-decanol as a modifier. Scrubbing with water removed the co-extracted iron. Finally, tungsten was recovered as ammonium tungstate (NH4)2WO4, the precursor of ammonium paratungstate, by stripping in a mixture of aqueous ammonia and ammonium chloride. Paratungstate is the most common intermediate for the production of tungsten oxide or tungsten metal. One drawback that needs to be adequately addressed, prior to further upscaling of this conceptual flowsheet, is the potential formation of trace levels of 2-chloroethanol in the leaching stage. It is hypothesized that this problem can be circumvented by further optimizing process conditions to enhance the mass transfer and reduce the reaction time, such as better mixing of the solid and the lixiviant.

Probing underlying mechanisms for PCBN tool decay during friction stir welding of nickel-based alloys

Friction stir welding (FSW) of nickel-based alloys can provide joints with improved corrosion resistance and mechanical properties that are deteriorated significantly during fusion welding of these alloys. However, rapid wear and poor longevity of tools are major concerns for FSW of nickel-based alloys. Polycrystalline cubic Boron Nitride (pcBN) has emerged as an alternative tool material due to its enhanced strength and stiffness but its use for FSW of nickel-based alloys is rarely explored. An investigation is therefore undertaken to examine FSW of Inconel 625, which is a commonly used nickel-based alloy, by experimental investigation and computational process modelling. The potential failure mechanisms of pcBN tools during FSW of Inconel 625 are examined. The results showed that the thermal softening of the tungstenrhenium binder phase and dissolution of tungsten into Inconel 625 are potential mechanisms to early wear and failure of the pcBN tools for FSW of Inconel 625.

Electrochemical Separation of Molybdenum and Tungsten Using Aqueous-Organic Electrolytes

Molybdenum is one of the valuable metals for the industry; its special properties make it extremely urgent to study the process of separation of molybdenum from other impurities. The article considers the optimization of electrochemical separation of molybdenum from Mo-W system. The electrochemical dissolution of molybdenum and tungsten in solutions of LiCl and NH4NO3 in dimethylsulfoxide was studied using polarization curves and calculation of the efficiency of anodic dissolution of molybdenum in the presence of tungsten. The electrolyte with a composition of 0.5 M LiCl; 5.2 M dimethylsulfoxide; 32.2 M water was selected as an effective solution for the electrochemical separation of molybdenum in the potential range of 1.0‒2.2 V. Results obtained in this study can be used for the development of selective separation method in the molybdenum production.

Theoretical analysis of mass transfer during anodic dissolution of tungsten rotating disk electrode in alkaline solutions

The mass transfer during the electrochemical dissolution of metals with the participation of electrolyte anions and the formation of stable anionic complex is analyzed by the example of tungsten anodic dissolution in the alkaline solutions. The distributions of ionic concentrations, partial current densities of ions and potential over the solution layer near the anode are determined as a result of numerical solution of the problem. The results of numerical solution are compared to the results of analytical solution and experimental data available from the literature. It is shown that the numerical solution of the problem with regard for migration, convection and concentration dependences of transport properties of solution agrees well with the experimental data. A complex role of ion migration in this process is revealed.

Improved Electrodialysis Synthesis of Perrhenic Acid from the Electrolytes of Processing the Wastes of Tungsten–Rhenium Alloys

The voltage drops across the elements of a three-chamber electrolytic cell are measured during the synthesis of perrhenic acid from potassium perrhenate, and the main contribution to the total bath voltage is shown to be made by the central chamber solution because of the low solubility of potassium perrhenate. The direct electrodialysis recovery of rhenium from the alkaline electrolytes formed during the anodic dissolution of tungsten–rhenium alloys is found to be unreasonable because of a rapid decrease in the process rate and high electric power consumption. The cleaning of tungsten–rhenium electrolytes from tungsten for the subsequent recovery of rhenium from a solution in the form of perrhenic acid is tested. The application of this technological sequence is shown to decrease the electric power consumed for electrodialysis fivefold and to decrease the rhenium losses die to the elimination of the stage of precipitation of potassium perrhenate. A fundamental technological scheme of the process is proposed.

Effect of Fe Addition to Binder Phase on Mechanical Properties of Tungsten Heavy Alloy

In the present work, tungsten heavy alloys (WHA) in two different compositions (92 wt% W–2:1 wt% Ni/Co and 92 wt% W–2:1:1 wt% Ni/Co/Fe) were synthesized through liquid phase sintering followed by swaging and thermal treatment. Microstructure and mechanical properties were analyzed to study the effect of the binder composition on the WHA. The W–Ni–Co alloy has lower matrix volume fraction, finer tungsten particle size and low contiguity when matched to the W–Ni–Co–Fe alloy. Mechanical investigations such as tensile, Charpy and hardness were tested and were correlated with the microstructural features. W–Ni–Co alloy showed higher strength, hardness and impact due to more solubility of tungsten in the binder and low contiguity. Hence, Fe addition to binder has resulted in degradation in required properties. For further enhancement in strength, aging has been carried out at 500 °C for 1 h and improved strength is obtained at the marginal expense of ductility. W–Ni–Co alloy has a much higher Charpy impact value of 127 J when compared to alloy with Fe addition (55 J) due to lower dissolution of tungsten into the binder matrix.

Electrochemical Properties of Tungsten in Molten Alkali Metal Chlorides

The anodic dissolution of metallic tungsten in the eutectic 3LiCl–2KCl (723–1023 K) and NaCl–2CsCl (823–1023 K) melts was studied by high-temperature electrochemistry and spectroscopy. W(IV) ions are formed in the melt. The conventional electrode potentials of tungsten were determined.

Investigation of the electrochemical dissolution behavior of tungsten during electrochemical machining

With the advantageous properties of a high melting point, high strength at high temperatures, low vapor pressure, and good thermal conductivity, tungsten is widely used in several advanced industrial applications. Micro structures such as micro holes and grooves are commonly prepared on the tungsten surface to enhance its heat transfer performance. Electrochemical machining (ECM) is a promising method of generating micro structures on the metal surface. Typically, an alkaline solution is employed as an electrolyte during tungsten ECM. Unfortunately, significant stray corrosion commonly occurs on the tungsten surface when high-concentration alkaline solutions (NaOH > 1 wt.%) are used. However, the machining efficiency decreases sharply when the concentration is less than 0.5 wt.%. In order to improve machining efficiency and accuracy, this paper proposes a novel mixed electrolyte consisting of NaClO3 mixed with a low concentration of NaOH for electrochemical machining of tungsten. The NaClO3 is used to increase the conductivity as well as to improve the formation of oxide films on the tungsten surface in order to avoid stray corrosion. NaOH is used to ensure continuous machining by removing the oxide film in the region being machined. The influence of the electrolyte composition and concentration on electrochemical dissolution of tungsten is analyzed by measuring the anode polarization curve and open circuit potential, and an optimized electrolyte with of 10 wt.% NaClO3 and 0.3 wt.% NaOH is identified. This optimized electrolyte enables generation of micro-through holes with inlet and outlet diameters of 500 and 410 μm, respectively, using a tube electrode with a diameter of 250 μm. The feed rate of the tool reaches 24 μm/min, which is 4 times faster than in previous studies.

Electrochemical stability of hexagonal tungsten carbide in the potential window of fuel cells and water electrolyzers investigated in a half-cell configuration

Tungsten carbide has attracted much interest as possible support for oxygen reduction and hydrogen oxidation in fuel cells and as catalyst itself for the hydrogen evolution reaction in water electrolyzers in the last years. Herein, we investigate the dissolution behavior of hexagonal tungsten carbide in acidic media with cyclovoltammetric and galvanostatic procedures under steady-state and dynamic conditions. The tungsten dissolution rate in the electrolyte was monitored in-situ and time resolved via coupling of the scanning flow cell with an inductively coupled plasma mass spectrometer (SFC-ICP-MS), allowing a direct correlation of potential and amount of dissolved species. The stability and passivation behavior of tungsten carbide was compared to pristine tungsten metal and its highest oxide WO3 in fuel cell/electrolyzer relevant potential ranges. It was found that partial passivation in the oxygen reduction region takes place, accompanied by steady dissolution of tungsten slightly above these potentials. In the HER/HOR region, no significant dissolution was observed. The dissolution rate of WC at high potentials was found to be in many cases almost one order of magnitude lower than for the pristine metal, yet two orders of magnitude higher than for its corresponding highest oxide.

Effect of multi-walled carbon nanotube addition on the microstructures and mechanical properties of Ti(C,N)-based cermets

The surface modification of multi-walled carbon nanotubes (MWCNTs) was carried out using plasma treatment. The microstructures of the prepared cermets with different additions of MWCNTs were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and X-ray diffraction (XRD). Mechanical properties such as transverse rupture strength (TRS), fracture toughness (KIC), and hardness (HRA) were measured. The results showed that some reactive groups were successfully modified on the surface of MWCNTs, resulting in the improvement of dispersibility. Most of the amorphous carbons and impurities were peeled from the MWCNTs. Increasing MWCNT addition decreased the dissolution of tungsten, titanium, and molybdenum in the binder phase. The cermet with 0.5 wt% MWCNT addition showed the highest TRS and fracture toughness. The strengthening mechanisms were attributed to the finer grain size, the homogeneous microstructure, and the higher volume fraction of binder phase in the binder. The toughening mechanisms were characterized by bridging and pulling-out.

Influence of partial pressure of water vapor on anodic dissolution of tungsten from super hard alloy tools in molten sodium hydroxide

As part of an ongoing study of a new tungsten recycling process using a molten sodium hydroxide bath, in which tungsten is anodically dissolved in the melt from tungsten-containing secondary resources, the influence of the partial pressure of water vapor on the electrochemical behavior of tungsten, cobalt, nickel, and tungsten carbide was investigated. Cyclic voltammograms of cobalt suggested that the solubility of cobalt compounds increased with the partial pressure of water vapor, whereas tungsten and tungsten carbide underwent no significant changes during their oxidation step. In addition, the current attributable to the evolution of hydrogen gas was measured when the partial pressure of water vapor became higher than 0.5atm. On the basis of the abovementioned results, anodic dissolution was carried out at a constant cell voltage using a super hard alloy tool. When water vapor was introduced into the system, the cell voltage and period required to dissolve the entire tip were reduced, even though the applied cell voltage was less than half that required under a pure argon atmosphere. These results indicate that the introduction of water vapor has the potential to improve the anodic dissolution step in the tungsten recycling process using a molten sodium hydroxide bath.

Investigation of erosion-corrosion behavior of (WTi)C based weld overlays

Erosion-corrosion is responsible for severe damage and high maintenance costs in many industrial operations involving slurry handling such as oil sands and mineral processing. While tungsten carbide reinforced Ni-based alloy overlays are frequently used for applications requiring extremely high wear resistance, they tend to suffer from extensive carbide degradation during the weld cladding process which has been shown to severely impair their erosion-corrosion resistance. Recently, new (WTi)C based overlays are being developed to alleviate some of the limitations associated with the nickel-based tungsten carbide overlays. In this study, three different overlays reinforced with (WTi)C and one tungsten carbide were assessed using a slurry pot erosion-corrosion testing apparatus where the total erosion-corrosion (E-C) rate as well as the separate components of synergistic effect was determined. Erosion-corrosion test was performed at 30°C, in an aqueous slurry containing 35wt% AFS 50–70 silica sand and 3.5wt% NaCl. A special SEM technique was employed to examine changes on the specimen surface at specific overlay locations before and after erosion-corrosion test in order to identify the effect of corrosion on erosion and vice-versa. It was found that tungsten is more prone to dissolution in the matrix during the welding process than titanium, which forms W-depletion/Ti-rich layer around the (WTi)C particle. Dissolution of tungsten and titanium promotes the formation of secondary carbides in the matrix. Erosion-corrosion performance of the overlays largely depends on increased fraction of retained primary (WTi)C phase.

In situmonitoring of the electrochemical dissolution of tungsten

In the present work, which is aimed to monitor in situ the electrochemical dissolution of tungsten by using a Flow-Type Scanning Droplet Cell Microscope (FT-SDCM) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), novel results are reported. The anodic oxide growth and its dissolution on the surface of W have been monitored in situ. The results of this current study show the importance of coupling electrochemical experiments to ICP-MS.